I obtained a key and retested on a Mac with El Capitan and it seems fine, so there is something else as the root cause.

One thing we have found is that DiffMerge will accept anything in that registration area and not check it until it's time to start up. Double check your key to make sure it is perfectly matched to what you received.

I checked the registration key, and it is exactly correct. I noticed there is a little button next to Registration in the menu that says Register. I pushed it and the popup I kept getting about registration stopped happening. Although, when I clicked it, I got the same popup I was always getting before. I had a conflict this morning, and the popup did not show. I'm hoping the issue is resolved.

If that was the fix, I think it would be better when we input the registration code, to have it automatically check itself. Having to click Register is a little odd from a UI standpoint.

Did you register under the same email that you have tied to your forum account here? If so, I'll check out the key to see if I can see anything wrong with it. If I don't see anything there, then we'll take this offline so I can take a closer look.